This invention relates to a new process for the production of synthetic resins containing isocyanurate groups by the polymerization of certain polyisocyanates of the diphenylmethane series which are described in more detail below. If desired, the reaction mixture used for producing these synthetic resins may also contain polymerizable monomers, organic epoxides or subequivalent quantities of compounds which contain isocyanate-reactive hydrogen atoms.
It is known that synthetic resins containing isocyanurate groups may be produced by the trimerization of the isocyanate groups of polyisocyanates in the presence of ethylenically unsaturated compounds (German Offenlegungsschrift No. 2,432,952) or in the presence of epoxides (German Offenlegungsschrift No. 2,359,386). The polyisocyanates which have been proposed for this purpose include, inter alia, diphenylmethane-4,4'-diisocyanate or mixtures of this diisocyanate with higher nuclear polyisocyanates of the diphenylmethane series. One disadvantage of using diphenylmethane-4,4'-diisocyanate is that the mixtures obtained are insufficiently reactive so that solid products are obtained only at high temperatures. Side reactions accompanied by the evolution of carbon dioxide which causes bubbles and blistering of the molded articles cannot be completely suppressed. The high temperatures required for hardening cause severe shrinkage in the cast products.
The same difficulties occur when commercial mixtures of diphenylmethane-4,4'-diisocyanate with higher nuclear polyisocyanates of the diphenylmethane series are used. The mixtures are relatively inert and require high temperatures for hardening. This causes high shrinkage in the cast product. Side reactions, accompanied by the liberation of carbon dioxide, cause gas bubbles in the cast products. Moreover, the resultant products are brittle. The viscosity of the starting materials is relatively high, making any impregnation more difficult and having a deleterious effect on the uptake of any filler.
It was therefore an object of the present invention to provide a new process for the production of heat-resistant synthetic resins, which process would not have the disadvantages mentioned above.